Election officials are braced for the biggest turnout in US history today as voters finally deliver their verdict on Barack Obama and John McCain to bring to an end a gripping, two-year campaign.

Although officials expressed confidence that polling booths would cope, campaigners and analysts expressed fears that the strain could see long queues and stations having to extend opening hours into the night. The effect would be a delay in declaring results in key states.

With all the main polls putting Obama well ahead, political analysts from right and left said they expected him to easily reach the 270 of 538 electoral votes needed to win the presidency, and many predicted a landslide, with him taking 350 or more electoral seats.

The Washington-based Pew organisation, one of the most respected pollsters which accurately predicted the vote in 2004, yesterday put Obama on 52% and McCain on 46%. RealClearPolitics, a website that averages out the main polls, put Obama on 51% to McCain's 44%. If the polls are borne out today, he would become the first Democrat since Jimmy Carter in 1976 to win 50% or more of the popular vote.

The excitement generated by Obama's candidacy is expected to see between 130 million and 140 million Americans vote, easily eclipsing the 121 million who turned out in 2004.

Obama and McCain both began multi-state tours yesterday in Florida, before heading towards their home states today. There was a poignant start to Obama's tour when he learned of the death of his 86-year-old grandmother, Madelyn Payne Dunham, while he was campaigning in Florida. She helped raise him and in a campaign advertisement this year he described her as having "taught me values straight from the Kansas heartland".

Obama decided to continue with his appearances and later told a rally in Jacksonville, Florida: "After decades of broken politics in Washington, eight years of failed policies from George Bush and 21 months of a campaign that has taken us from the rocky coast of Maine to the sunshine of California, we are one day away from change in America."

Obama was unable to avoid providing occasional glimpses throughout the day that he expected to become the 44th president. In a courteous gesture he might not have made if the race was closer, Obama said he wanted to "congratulate McCain on the great race that he has fought".

Exuding confidence, he told a rally the previous night: "The last couple of days, I've been just feeling good."

As polling day nears, he has become more reflective, telling CBS yesterday that he had found the toughest part of the two-year campaign was the rightwing attack on his wife, Michelle. He described this as "just completely out of bounds".

McCain, at a rally in Tampa, Florida, insisted he could still pull off a surprise win. "The pundits may not know it and the Democrats may not know it, but the Mac is back. We're going to win this election."

The Democrats, as well as political analysts, were less sanguine than election officials about trouble-free voting, saying that with 300,000 precincts across the country, there would be problems.

Democratic officials complain that in some states Republican election officers are not opening enough booths in areas that are predominantly African-American in the hope that some voters, put off by queues, will go home without voting.

Professor Michael McDonald, an election expert at Virginia's George Mason University, said: "I would not be surprised if we had long lines, potentially keeping polling places open longer on election day." He anticipated problems in states that had not allowed early voting in significant numbers, such as Virginia, Pennsylvania and Missouri.

The Democrats are outspending the Republicans on final-day advertising by at least two to one. They are also likely to have more staff and volunteers out, with 770 offices nationwide to McCain's 370.